Analog signal to frequency converters are well known. Analog signals generated by various devices such as sensors are often converted into corresponding digital signals because of the convenience and accuracy of digital signal processing. Often the sensor is remote from the computer, therefore, the signal has to be transmitted to the computer. Accurate signal transmission with analog signals is very difficult. In order to improve the accuracy, the signal is converted into a frequency by means of an analog signal-to-frequency converter. In the digital form, the signal can be transmitted to the computer substantially without interference. In the computer, a digital word is formed out of the frequency, for example, by counting the pulses during a predetermined time. This type of transmission offers the advantage that the frequency is not disturbed by the attenuation of a transmission cable or similar influences.
A well known type of converter is the charge balanced voltage-to-frequency type. This type of converter generally includes a capacitor, connected with an operational amplifier to form a current integrator that is cyclically charged, first in one direction, second in the opposite direction (i.e., charged and discharged). This is done at a frequency which changes linearly with the input voltage applied from the sensor. The net charge applied to the capacitor during each cycle is zero, a result achieved by charging the capacitor in the first direction for a predetermined period of time. In response to the end of that time, the capacitor is charged in the opposite direction until a predetermined level is reached. The rate at which the capacitor is charged in the first direction is controlled by the sum of a current derived from the input voltage and a fixed current source. The rate at which the capacitor is discharged is determined by a current derived from the input voltage. Therefore, the frequency of the charge and discharge cycles is a direct function of the input voltage magnitude.
In another charge rebalancing scheme, an input current from the sensor builds up a charge on the capacitor for the integrator. The comparator output initiates a feedback circuit which transmits a pulse of current of the opposite polarity of the input signal to the capacitor. The number of pulses required to balance the capacitor charge is indicative of the magnitude of the sensor signal. A bit is added to the digital word output to indicate polarity of the incoming signal.
Conventional integrating current to frequency converters have not been capable of providing performance goals of linearity, symmetry and bias stability required in critical applications. One reason for the lack of accuracy of such converters is that the negative and positive current sources are unequal, or that consecutive rebalance pulses do not contain like energy. Also, bias voltages which build up across the integrator affect the accuracy of the converter. Another factor which affects the accuracy is the great amount of switching of the rebalancing current sources. In most cases, switches either absorb or add energy to the rebalance current in a non-symmetrical manner. This has an adverse effect on the linearity of the converter. In addition, the resolution of these converters is proportional to the rate at which the integrator charge is balanced. Since the non-linearity is also proportional to this rate, there is a limit to the resolution achievable.